There are mainly three known types of mechanism for driving a calendar indicator, in particular a date ring: a continuous mechanism, a conventional semi-instantaneous mechanism and an instantaneous mechanism.
In a conventional variant shown in FIG. 1, the continuous mechanism 2 is arranged to drive a date ring 4 provided with an inner toothing 6. This continuous mechanism includes a Maltese cross 8 and a wheel set 12 for actuation thereof. The Maltese cross includes six branches 9 and is integral with a coaxial pinion 10 which meshes with the date ring toothing 6. Pinion 10 has six teeth. The number of branches of the Maltese cross and the number of teeth of the pinion are given here by way of non-limiting example. Thus, there is also a known mechanism of this type having a Maltese cross with four branches and a pinion with eight teeth. Preferably, the ratio between the number of teeth and the number of branches is an integer number. Actuation wheel set 12 includes two drive pins 16, 17 and a locking member 14 which cooperates with branches 9 to lock the Maltese cross in stable positions between two successive drive operations respectively performed by the two pins. This actuation wheel set is driven in rotation, for example, by a pinion 20. As the operation of a Maltese cross system is well known, it will not be described in more detail here.
The continuous drive mechanism described above is characterized by a gear with little or no play, and by the absence of a jumper spring. Thus, the driving function and the function of positioning the date ring in its display positions are both performed by the pinion associated with the Maltese cross. Moreover, the shock-resistant function is performed by the Maltese cross system, as the locking member 14 easily ensures this function. The manufacture of a timepiece movement with this type of mechanism is expensive since it it necessary to reduce to a maximum the machining and assembly tolerances (manufacturing tolerances) of the mechanism and of the date ring, to ensure precise positioning of the ring in its display positions, for example precise centering of each date in the aperture of a dial provided for the timepiece movement.
The conventional semi-instantaneous drive mechanism includes a drive wheel for the date ring generally provided with one finger-piece or two finger-pieces which periodically penetrate(s) the ring toothing to drive it from one display position to the next. The spaces between the teeth of this toothing are generally made relatively wide, in particular to allow each finger-piece to enter and exit the ring toothing with no risk of locking; especially because of the manufacturing and centering tolerances of the date ring. Thus, it is clear that the drive wheel cannot ensure the function of positioning the ring. Further, it cannot ensure a shock resistant function, since generally there is no meshing between the toothing and the finger-piece, respectively the two finger-pieces, over a certain range of angles of the drive wheel.
To ensure the positioning function and the shock resistant function, there is provided a jumper-spring, also called a “jumper”, which is inserted between two successive teeth of the ring toothing. This is referred to as a semi-instantaneous system since, in a first phase of the change to the next date, the ring is driven in rotation by a finger-piece of the drive wheel and the tip of the tooth downstream of the jumper lifts the jumper until the tip of the jumper is resting against the tip of the tooth. Next, the jumper exerts a tangential force on the rear flank of the tooth concerned and then takes its next rest position. In this second phase, the jumper rapidly drives the date ring in rotation to its next display position, the finger-piece of the drive wheel continuing its rotation at a lower speed than the ring and thus ceasing to exert a torque force on the ring. The space between the teeth is arranged to be sufficiently large so that the tooth following the tooth pushed by a finger-piece does not abut against the finger-piece inserted into the ring toothing. A major drawback of this semi-instantaneous mechanism arises from the fact that the shock resistant function is accomplished by the jumper, which therefore has to press on the date ring with significant force in order to exert sufficient locking torque in the event of a shock. Thus, at each change of date, the drive mechanism must provide a large drive torque to overcome the positioning torque of the jumper; which requires a great deal of energy and a mechanism capable of providing such a drive torque at the date ring toothing.
There exist several embodiments of an instantaneous drive mechanism. In each case there is provided a positioning jumper that also ensures the shock resistant function. This mechanism therefore has the same drawbacks as the semi-instantaneous mechanism described above.